The present invention relates to a variable valve performance apparatus for varying performance of a set of intake valves or a set of exhaust valves provided on a camshaft of an engine by moving cams that have changing cam profiles along the axis of the camshaft. More particularly, the present invention pertains to an improvement of an apparatus that feedback controls the movement of cams based on the position of the cams detected by a sensor.
Many existing engines are equipped with an apparatus that varies the performance characteristics of a set of intake valves or a set of exhaust valves as necessary. This enhances the power and performance of the engine and reduces undesirable emissions. Japanese Unexamined Patent Publication No. 4-187807 discloses such a variable valve performance apparatus. FIG. 17 is a schematic view of this apparatus.
As shown in FIG. 17, a variable valve performance apparatus 101 is provided on a cylinder head 102 of an engine. The cylinder head 102 has an intake passage 103, which is connected to a combustion chamber (not shown). An intake valve 104 is located in the intake passage 103. The valve 104 selectively connects and disconnects the combustion chamber with the passage 103. The mechanism 101 varies the performance of the valve 104.
The apparatus 101 includes a camshaft 105, a cam 106 and a cam moving mechanism 107 located above the cylinder head 102. The cam 106 is located on the camshaft 105 and is moved along the axis of the camshaft 105 by the cam moving mechanism 107.
The camshaft 105 is rotated by a crankshaft (not shown) of the engine. The cam 106 contacts the upper end of the valve 104. The cam 106 slides along the axis of the camshaft 105 and rotates integrally with the camshaft 105. The profile of the cam 106 continuously changes along the axial direction of the camshaft 105 as shown FIG. 18.
When rotating integrally with the camshaft 105, the cam 106 causes the intake valve 104 to open and close. Moving the cam 106 in the direction of arrow Q gradually advances the opening timing of the valve 104, prolongs the duration of opening of the valve 104 and increases the lift of the valve 104. Moving the cam 106 in a direction of an arrow P gradually retards the opening timing of the valve 104, shortens the duration of opening of the valve 104 and decreases the lift of the valve 104.
Such changes of valve performance are usually performed in the following manner. When the engine speed is low, the opening duration of the valve 104 is shortened and the lift of the valve 104 is decreased for stabilizing the engine speed and for enhancing the engine torque. As a result, the speed of air-fuel mixture, when being drawn into the combustion chamber, is increased. When the engine speed is high, the opening duration of the valve 104 is prolonged and the lift of the valve 104 is increased for increasing the power of the engine. As a result, the amount of air-fuel mixture drawn into the combustion chamber is increased.
The cam moving mechanism 107 includes an arm 108, a screw rod 109 and a control motor 110. The arm 108 holds the cam 106 by contacting the cam 106 at both ends. The screw rod 109 extends parallel to the cam shaft 105. The arm 108 is screwed on the rod 109. The control motor 110 rotates the rod 109 thereby causing the arm 108 to move along the rod 109. Accordingly, the cam 106, which is held by the arm 108, moves along the axis of the camshaft 105.
The control motor 110 is controlled by a controller 111. The controller 111 receives detection signals from an engine speed sensor 115 and other sensors 112 that detect the running state of the engine. The controller 111 controls the motor 110 based on the inputted signals. The controller 111 also receives detection signals from a cam position sensor 116, which detects the position of the cam 106, and feedback controls the motor 110 based on the signals indicating the cam position. As a result, an optimum cam profile is selected in accordance with the running state of the engine.
A gap sensor may be used as the cam position sensor 116. The gap sensor includes a coil that generates induced electromotive force in accordance with the position of a cam. The gap sensor outputs the generated induced electromotive force as a detection signal. However, the gap sensor has a relatively limited range of accurate position detection. It is therefore difficult for the gap sensor to accurately detect the position of the cam along the entire movable range of the cam.
The gap sensor may be replaced with an optical sensor. However, engine vibrations and grime degrade the detection accuracy of the optical sensor. Thus, the optical sensor is not reliable.
If the position of a cam is not accurately detected, it is difficult to optimize the cam profile in accordance with the running condition of the engine.